Director valve based voltage source converters have recently been developed where there is a string of director valves connected in series between two Direct Current (DC) poles and a string of multilevel cells connected in parallel with this valve string. In this structure the director valves are used for directivity and the multilevel cells for waveshaping. A voltage source converter where the director valves are thyristors is for instance disclosed in WO2014/082657.
The structure may furthermore be used with all three Alternating Current (AC) phases provided through three such valve strings connected in series between two DC poles and three multilevel strings also connected in series between the two DC poles and in parallel with the valves.
In this latter structure there may be a number of problems that may occur.
If the AC voltage varies when the multilevel cells are used, then also the average DC voltage is changed. The AC and DC voltages are coupled to each other.
Furthermore, AC fault ride-through is not possible if AC voltage and DC voltage are tightly coupled.
The DC voltage from the converter furthermore comprises 6n harmonics which can be eliminated by passive filters. However, passive DC filters are bulky and occupy a lot of space. It may therefore be of interest to remove and/or limit the size and complexity of such DC filters.
The invention addresses one or more of the above mentioned problems.
With regard to this it would be of interest to provide an improved voltage source converter and especially one where the DC voltage is decoupled from the AC voltage.